1. Field of the Invention
The present invention relates to a clamp apparatus capable of clamping a workpiece by using an arm which is rotatable by a predetermined angle in accordance with a driving action of a driving source.
2. Description of the Related Art
The clamp apparatus has been hitherto used, for example, in order to clamp a constitutive part when the constitutive part of an automobile or the like is welded. Such a clamp apparatus is disclosed, for example, in U.S. Pat. No. 4,723,767.
In the clamp apparatus disclosed in U.S. Pat. No. 4,723,767 (not shown), a ball screw shaft is linearly displaced in accordance with the driving action of a motor. The rectilinear motion of the ball screw shaft is converted into the rotary motion of an arm by the aid of a link mechanism.
A switch rod, which is displaceable integrally with the ball screw shaft, is coaxially connected to one end of the ball screw shaft. The position of rotation of the arm is detected by sensing the switch rod by using a pair of switch elements which are separated from each other by a predetermined spacing distance.
However, the clamp apparatus concerning the conventional technique described above involves the following inconvenience. That is, the position, at which the arm is subjected to the clamping, differs, for example, depending on the thickness of the workpiece. Therefore, when the angle of rotation of the arm is changed, the distance of the linear displacement of the switch rod differs. It is impossible to sense the switch rod by using the pair of switch elements.
In other words, the following inconvenience arises. That is, each of the pair of switch elements senses the switch rod when the switch rod makes approach. Therefore, when the amount of rotation of the arm is changed, the switch rod does not make approach up to be within an allowable range in which the switch rod is sensed by the switch element.
A general object of the present invention is to provide a clamp apparatus which makes it possible to reliably detect the position of rotation of an arm even when the amount of rotation of the arm is changed.
A principal object of the present invention is to provide a clamp apparatus which makes it possible to reliably detect a first terminal end position of displacement to give an unclamping state and a second terminal end position of displacement to give a clamping state for an arm, by selecting an arbitrary detection object from a plurality of detection objects having different lengths respectively, corresponding to an amount of rotation of the arm.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.